1. Field of the Invention
The invention is directed to a stabilizer for drilling the straight portion of a deviated well. More particularly, the invention is directed to a plurality of stabilizers mounted on a single mandrel for use with flexible drill pipe in drilling the straight portion of a deviated well.
2. Prior Art
Directional drilling has become a routine development operation throughout the world. In general, it may be defined as the art of controlling a rotary drill's directional and angular tendencies of penetrating the earth's surface to a special subsurface target. Much technology has been developed in order to insert a drill string into a deviated hole and then to drill an essentially straight bore hole deviated from the perpendicular.
One method of accomplishing this is to use flexible drill pipe which can literally bend around a short curved radius. The flexible drill pipe is capable of withstanding the compressive and rotational forces exerted in forcing the bit to drill a bore hole. Typical of the efforts to develop such flexible drill pipe is U.S. Pat. No. 2,515,366 (issued to J. A. Zublin).
In using a drill bit connected directly to such flexible drill pipe, in attempting to drill the straight portion of a deviated hole, there is a tendency for the bit to spiral in the hole, to drift from the desired azimuth, develop dog-legs and in general drill anything but a straight hole.
Over the years there has been developed a variety of stabilizers that are affixed to drill collars to aid in drilling a straight hole. Minimum bit efficiency is achieved when various arrangements of stabilizers are used in conjunction with their proper spacing from the drill bit.
A typical make up of drill bit, stabilizers and drill collars is illustrated on page 1826 of the Composite Catalog of Oil Field Equipment and Services, 34th Ed., Vol. 1, 1980-81. There are generally two types of stabilizers used in drilling today. There are the rotating and non-rotating types. Typical of the non-rotating type of stabilizer is that manufactured by Drilco, a division of Smith International, Inc. Their non-rotating sleeve-type stabilizer is illustrated on page 2511 of the 1980-81 Composite Catalog.
These stabilizers are typically used as a single unit mounted on a mandrel which is made up in the drill string just above the drill bit. It is essentially uniformly recommended that if additional stabilizers are to be used, they should be separated by a length of drill collar.
It has been found, however, that the art of stabilizers has been developed using rigid drill pipe. In fact, drilling manuals devoted to this topic speak in terms of using a "stiff bottom assembly" to resist any change in direction of a hole. Thus, operators commonly use a stiff bottom assembly with a single stabilizer positioned adjacent the bit.
This technology has been found to be totally inapplicable to flexible drill string drilling the straight section of a deviated hole.